(703b) Tribo-Charging of Binary Mixtures Composed of Coarse and Fine Particles in Gas-Solid Pipe Flow

Tribo-charging
of Binary Mixtures Composed of Coarse and Fine Particles in Gas-Solid Pipe Flow

Farzam Fotovat^1,2*, Haifeng Wang^1,3,
Xiaotao T. Bi¹ and John R. Grace¹

¹Fluidization
Research Centre, Department of Chemical and Biological Engineering,

University of British
Columbia, Vancouver, Canada V6T 1Z3

²Department
of Chemical and Petroleum Engineering, Sharif University of Technology,

Azadi Avenue, P.O.
Box 11365-9465, Tehran, Iran

³Department
of Chemical Engineering, China University of Mining and Technology,

Xuzhou, China, 221166

                             
1.     
Introduction

Despite
the studies carried out on tribo-charging of particle mixtures composed of fine
and coarse particles, it is not clear yet how fines alter the tribo-charging
behavior of coarse particles in gas-solid flows. Moreover, the effect of the
physical, chemical and electrical properties of particles on the
electrification of powdery mixtures has not been well understood, requiring further
investigation. In this study, tribo-charging of mixtures of coarse glass beads
and conductive or non-conductive fines of different compositions is
investigated in a pneumatic conveying system. Understanding the relationship
between charge density and composition of particle mixtures, as well as determining
the impact of fine particle properties on electrification of binary mixtures of
fine and coarse materials are the key objectives of this investigation.     

 
2.     
Modeling electrification of binary
mixtures in dilute gas-solids pipe flow

According
to the model developed by Matsusaka & Masuda ¹ based
on a condenser model ^2,3,the
mass charge density of an initially neutral single spherical particle charged
in a dilute gas-solids pipe flow is

where S and z₀
are the contact area and the critical gap between the contact bodies (particle
and pipe wall in our case). ρ_p and d_pare
the particle density and diameter, respectively. n denotes the number of
particle-wall collisions, and ε₀ is vacuum permittivity (8.854×10⁻¹²
F/m). k_c is the charging efficiency and V_c
is the potential difference based on the surface work functions of contacting
bodies:

where ϕ_p
and ϕ_w are the work functions of the particle and pipe
wall, respectively, and e is the elementary charge. Note that the
potential differences arising from space and image charges are assumed to be negligible
in the systems studied here. It is also assumed
that V_c of a binary mixture is obtained from the
superposition of the potential differences between fine/coarse particles and
the pipe wall, i.e. V_cf and V_cc,
respectively:

To
adapt Eq. (1) to a binary mixture of dissimilar particles, we assume that n
is proportional to the total number of particles in the mixture:

where
k is the proportionality factor. N_f, and N_c
are the number of fine and coarse particles in the unit mass of the mixture,
respectively, calculated as follows:

where
x is the weight fraction of fine particles in the mixture. The c
and f subscripts denote the coarse and fine particles, respectively. From
Eqs. (1) and (3-6), the net charge of all fine and coarse particles in a unit
mass of a binary mixture is given by:

The
coefficient of Eq. (7) can be obtained by fitting the equation with the measured
mass charge densities of the mixtures.

 
3.     
Experimental
3.1.  
Materials

Glass beads were chosen in this study as the coarse
material for all mixtures. Conductive silver-coated fine glass beads (SFGB) and
uncoated dielectric fine glass beads (FGB) were used as fine materials added to
coarse glass beads (CGB). These fine materials were similar in terms of mean
size, size distribution, and density. Table 1 provides the key properties of CGB
and fines used in this study.

 
3.2.  
Charge measurements

The electrostatic test system used in the present work
consisted of a cylindrical feeder, a grounded stainless steel spiral tube (3.0
m in length, 0.64 cm in diameter, 2 coils (d_s=0.15 m) per meter)
for tribo-charging, a Faraday cup for charge measurement, and control valves. Dry
building air was used as the particle-carrying gas. During the tests,
temperature and relative humidity were varied in the ranges of 16.9 to 25.7^oC
and 0.2% to 7.2%, respectively. Based on
the preliminary experiments carried out to obtain reproducible experimental
conditions, 1.0 g particle was conveyed in the pipe by air flowing with the
velocity of 16.5 m/s in all tests. The air inlet pressure was about 600 kPa.

In order to prepare 1.0 g mixtures consisting of 0.2,
1.0, 2.0, 5.0, 10.0, 20.0, 30.0, 70.0, and 90.0 wt.% fines, predetermined
amounts of fines and CGB were mixed together. In addition, 1.0 g of CGB and 1.0
g fines were tested in separate experiments. The charge measurements of each mixture and pure materials were repeated
at least three times.

 
4.     
Preliminary Results

Charge
densities of CGB-SFGB mixtures of various compositions are shown in Fig.
1. Unlike the negative polarity of pure
CGB, pure silver-coated fine glass beads were charged positively, consistent
with the negative V_cf value
obtained from Eq. (2), knowing the work functions of the stainless steel pipe
wall and SFGB provided in Table 1. Compared to the pure coarse particles, the
magnitude of charge densities of mixtures to which less than 10 wt. % of fines had
been added increased with increasing fines content, and the charge polarities remained
negative. With increasing weight
proportion of SFGB in the mixture beyond 10%, the charge polarity changed from
negative to positive and the charge density then slightly increased.

Fig.
2 depicts the effect of added fine glass beads (FGB) on tribo-charging of CGB. As
per SFGB alone, uncoated fine glass beads by themselves were charged positively,
despite the negative polarity of pure CGB. This shows that the fine and coarse
glass beads used in our study likely differed in chemical composition or
surface properties, leading to dissimilar contact potential differences with
the pipe wall.

Adding
FGB to coarse glass beads resulted in an increase in the absolute value of mass
charge density, relative to pure CGB. The charge polarity remained negative when
the weight percentage of fines was less than 10 wt.%. Further addition of fines
reversed the mixture polarity, and the charge density then leveled off. Despite
the lower magnitude of charge densities for CGB-FGB mixtures than for
corresponding CGB-SFGB mixtures, charge densities of both systems followed similar
trends when changing the weight fraction of fines from 0 to 1.

As seen in Figs. 1 and 2, |q_m|
experienced a local maximum at x ≈ 0.1, beyond which contacts between fine
particles and the pipe wall became the dominant charging mechanism, as inferred
from the change in the charge polarity. Thus, observation
of charge polarity reversal in gas-solid pipe flows involving mixtures of fine
and coarse particles can signify a change in the type of particles dominantly
in contact with the pipe wall. Figs. 1 and 2 depict that the measured
charge densities of mixtures with x > 0.1 were widely scattered, likely due
to the opposite charge polarities of fine and coarse particles and the major influence
of turbulence on the dynamics of fine particles, as discussed below.

Figs. 1 and 2 also suggest that
Eq. (7) could successfully predict the trend in the mass charge densities of
all tested mixtures as a function of the fines content. The discrepancy observed between the experimental results and the
proposed model in Figs. 1 and 2,
especially in the range with low fraction of fine particles, is likely
attributed to negligence of particle-particle interactions, which should be
further investigated in the future.

 
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